You have the 7 port which is a good all around motor giving you decent bottom end and an excellant top end. I personally only have stayed with the 3 for my buggy and the 5 for my truggy. If you maximized the motor with the 8 for power you could be just wasting your time if you can't plant the power to the ground. A good rule of thumb is if you can't control it or the rear end is kicking out, drop down in restrictor. One, it'll save fuel and two, it'll make the buggy more stable. Drag racing down the front straight doesn't mean a thing if you can't control it in the in field. Like the saying goes........slow is fast!!!
I normally keep a 5.5 in most my engines for the fuel economy and believe me it's just as fast as anyone down the straight. I honestly can't remember the last time I used anything over a 6.5, haven't needed to.